Bonding aluminum metals



United States Patent 2,987,816 BONDING ALUMINUM METALS Robert A, Noland, Chicago, and David E. Walker, Park Forest, 111., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Dec. 10, 1956, Ser. No. 627,517 Claims. (Cl. 29-493) This invention deals with the bonding of aluminum 'or aluminum-base alloys to aluminum or aluminum-base alloys. (Aluminum and aluminum-base alloys, which are alloys whose predominant component is aluminum, be referred to hereinafter collectively as aluminum metals.)

Aluminum metals have been joined to each other heretofore by roll-bonding at about 600 C. under a pressure that the reduction of thickness ranged from 65 to 70%. Without this high degree of reduction, a satisfactory bond could not be obtained because an oxide film formed which quasi functioned as a barrier and prevented integration during bonding. However, there are instances where a reduction is not desired and rollbonding therefore cannot be used. This is, for instance, the case in canning fuel elements for nuclear reactors where a cylindrical or otherwise elongated core containing the fissionable material is protected by an aluminum jacket or can. While the side part of the aluminum jacket can be applied by roll-bonding whereby, by the reduction of thickness, the dimensions desired are obtained, the end plates cannot be bonded to the core and to the jacket because a further reduction, which then would "be in the longitudinal direction, is not desirable.

It has proposed to apply the end plates by brazing, which requires the use of a flux; however, this, too, is undesirable in the particular case of fuel elements because flux materials usually contain elements of high cross-section, such as boron.

It is an object of this invention to provide a process of bonding aluminum metals to each other for which no flux is requiral.

It is another object of this invention to provide a proces of bonding aluminum metals to each other by which a satisfactory bond is obtained without any substantial dimensional reduction.

It is also an object of this invention to provide a process of bonding aluminum metals to each other in which any oxide film is removed as it is formed so that a welladhering bond is obtained.

It is finally also an object of this invention to provide a process of bonding aluminum metals to each other by which a continuous nonleaking joint is obtained.

These and other objects are accomplished by applying silicon powder to the surface of at least one of the two aluminum units to be joined, placing said other unit on said silicon-powdered surface to form an assembly, applying pressure at elevated temperature to said assembly whereby the two units are integrally bonded.

The silicon powder advantageously has a small grain size; particles of between -100 and -325 mesh have given the best results.

The silicon powder can be applied to one or both units to be joined, for instance by means of a sieve. After the other unit has been superposed, rubbing of the two units at their silicon-contacting surfaces at room temperature is advantageous. By this rubbing action the sharp edges of the silicon crystals rupture any oxide film. The assembly is then heated to 580 C. and at the same time placed under pressure. The silicon crystals alloy with the aluminum metal to form the aluminum-silicon eutectic, liquid at 580 C., and the eutectic is squeezed out from between the contacting surfaces of the units together with 2 any oxide film that had formed so that then a good joint between the oxide-free surfaces of the units of metals is obtained.

The silicon can also be applied in the form of a liquid suspension. For this a suspending medium is used that is nonreactive with the aluminum metals and with the silicon; it should also be volatile at the bonding temperature. Liquids found suitable for this purpose are water, alcohol, amyl acetate and a mixture of alcohol and glycerin, e.g. in a ratio of 50:50 percent by volume. The silicon suspension, of course, can be applied by any means known; application by brushing has been found simple and satisfactory.

As has bun indicated above in connection with the rubbing step, it is believed that the silicon has a dual function in the process of this invention. In the first place, the edges of its crystals break oxide film on the surfaces to be bonded. In the second place, the silicon forms a relatively low-melting alloy, probably the eutectic, with the aluminum metals which under the influence of pressure is squeezed out from between the units. This aluminum-silicon alloy at the same time drags along and thus removes the oxide film.

Articles produced by the process of this invention, when examined microscopically, showed no separation line at the interface, but looked like one integral body with the exception that a few islands of aluminum-silicon alloy are sometimes left embedded at the place of bondmg.

Any apparatus known to those skilled in the art can be used for the process of this invention. It was preferred by the inventors to place the assembled unit between two heated dies by which a pressure was then applied. The preferred pressure is above 400 p.s.i., but the best results were obtained with a pressure of between 800 and 900 p.s.i. The optimum temperature for bonding ranged between 550 and 580 C. The reduction accomplished under these conditions averaged about 5%.

It will be understood that, while the invention has been described by way of example in regard to the canning of fuel elements, it is applicable to the bonding of any other structural elements of aluminum metals and that it is particularly valuable wherever a continuous leakless bond is desired.

In the following an example of the process of this invention is given for illustrative purposes without the intention to have the invention limited to the details given therein.

Example Two pieces of aluminum metal, each 2 /2 5X%", were used. A suspension consisting of equal volumes of silicon powder (grain size of 325 mesh) and a glycerin-alcohol mixture (50:50% by volume) was painted on the surface to be bonded together of each piece, and each piece was then slowly dried on a hot plate until all of the glycerin-alcohol mixture had evaporated. Thereafter the two units were assembled in a sandwichlike manner with the two silicon-coated surfaces contacting each other. The assembly formed was then inserted in a heated die press, and a pressure of 11,500 lbs. (920 psi.) and a temperature of between 580 and 590 C. were applied thereto; the assembly was held at temperature and pressure for about 13 minutes. Thereafter the assembly was cooled to 500 C. while still under said pressure, and then the unit was removed from the die press.

The tensile strength of the bond was then measured by welding a cylindrical stud of aluminum perpendicularly to the side which is parallel to the interface of the finished product. Immediately around the stud the aluminum was then removed whereby a circular groove was formed the bottom surface of which was the interface.

chine and tested by applying a pulling force to the stud.

The bond broke at a load corresponding to a tensile strength of about 16,850 p.s.i.

Microscopic examination showedthat there was no separation line at the interface between the two original units, but that the product was one integral piece; the only'indication of the location of the bond was the presence of a few islands of aluminum-silicon alloy.

It will be understood that this invention is not to be limited to the details given herein and that it may be modified within the scope of the appended claims. What is claimed is:

1.'A process of bonding two units of aluminum base metal, consisting in applying silicon powder to the surface to be bonded of at least one of said units; placing the other unit on said surface whereby an assembly is formed; rubbing the two units against each other while at room temperature and the silicon is solid, whereby any oxide film on the surface is ruptured; and applying pressure and heat to said assembly whereby an aluminumsilicon alloy forms, is squeezed out from between the two units, the oxide film is dragged along by the aluminum silicon alloy and the two units are integrally joined.

2. The process of claim 1 wherein the assembly is heated to between 550 and 580 C. and the pressure is at least 400 p.s.i.

3. The process of claim 2 wherein the pressure ranges 6. The process of claim 5 wherein said medium is water.

7. The process of claim 5 wherein said medium is ethyl alcohol.

8. The process of claim 5 wherein said medium is amyl acetate.

9. The process of claim 5 wherein said medium is a mixture containing 50% by volume each of alcohol and glycerin.

10. A process of bonding two units of aluminum base metal, consisting in coating the surface to be bonded of at least one of said units with silicon powder at room temperature; assembling said two units at room temperature so that the silicon powder is in between said two units; placing the assembly formed into hot dies mounted in a press; and increasing the pressure on said assembly while its temperature is also raised whereby the solid silicon powder ruptures any oxide film on the surfaces of the units to be bonded and alloyswith aluminum to form the aluminum-silicon eutectic, the eutectic melts and is squeezed out from between the two units, and the aluminum oxide is removed together with eutectic.

References Cited in the file of this patent UNITED STATES PATENTS 549,610 Ramage Nov. 12, 1895 733,662 Lange July 14, 1903 781,338 Heil Jan. 31, 1905 1,050,880 Wadsworth Jan. 21, 1911 1,853,370 Marshall Apr. 12, 1932 2,100,257 Larson Nov. 23, 1 937 2,155,651 Goetzel Apr. 25, 1939 2,258,681 Hoglund Oct. 14, 1941 2,301,332 Scheller Nov. 10, 1942 2,317,510 Barklie et al Apr. 27, 1943 2,473,888 Jordan June 21, 1949 2,602,413 Miller July 8, 1952 2,733,168 Hodge et a1. Jan. 31, 1956 

1. A PROCESS OF BONDING TWO UNITS OF ALUMINUM BASE METAL, CONSISTING IN APPLYING SILICON POWDER TO THE SURFACE TO BE BONDED OF AT LEAST ONE OF SAID UNITS, PLACING THE OTHER UNIT ON SAID SURFACE WHEREBY AN ASSEMBLY IS FORMED, RUBBING THE TWO UNITS AGAINST EACH OTHER WHILE AT ROOM TEMPERATURE AND THE SILICON IS SOLID, WHEREBY ANY OXIDE FILM ON THE SURFACE IS RUPTURED, AND APPLYING PRESSURE AND HEAT TO SAID ASSEMBLY WHEREBY AN ALUMINUMSILICON ALLOY FORMS, IS SQUEEZED OUT FROM BETWEEN THE TWO UNITS, THE OXIDE FILM IS DRAGGED ALONG BY THE ALUMINUMSILICON ALLOY AND THE TWO UNITS ARE INTEGRALLY JOINED. 